Or they are using floo powder

I don't know, that's what Google told me was used to make purple fireworks

google told me the same but my experience says otherwise

chemistry.about.com says that a potassium salt would yield a purple-blue flame, and adding a little strontium will add more of the red to make it a better purple. But it also says to keep in mind that purple is one of the hardest colors for our eyes to see

I wonder if copper solution vs solid copper makes a difference

The combustion and the chemical reaction would be different

Or photoshop